Binary digital data is generally communicated from a transmission location to a receiving location by impressing a signal representing the data on a transmission path (which term is used herein to designate both a broadcast medium, such as air, and a transmission line, such as a metallic cable, a waveguide or an optical fiber guide) from the transmission location to the receiving location. Depending on the nature of the transmission path, the data signal may be transmitted in modulated or unmodulated, i.e., baseband, form. A periodic clock signal is used to establish a succession of unit intervals of nominally equal duration, within which the successive bits are transmitted. The clock signal is also used to recover the binary data at the receiving location. Thus, using the clock signal, the baseband data signal is sampled of the mid-point of each unit interval in order to determine whether the signal transmitted during that unit interval represented a binary 1 or a binary 0.
Naturally, it is important that digital data be transmitted over the transmission path without unacceptable distortion being introduced by a transmission path, so that the binary data can be accurately recovered at the receiving location. The most serious distortions introduced by a transmission path include high-frequency rolloff, i.e., the manner in which the attenuation factor of a transmission path increases as a function of the frequency of the signal being transmitted and the group delay distortion which is associated with high-frequency rolloff. For this reason, the data signal is configured to minimize its high-frequency components by avoiding use of rectangular transitions between binary 1's and binary 0's and causing the data signal to follow a predetermined non-rectangular trajectory, e.g., of sinusoidal or sine-squared form, which includes fewer high-frequency components than a rectangular signal trajectory.
It is well known to use an eye diagram to monitor the suitability of a transmission path between a transmission location and a remote receiving location for transmission of digital data. The eye diagram is generated by displaying the received data signal on an oscilloscope synchronized to the clock signal. The display is a representation of the eight possible signal trajectories through a unit interval and defines an area shaped somewhat as the human eye. The severity of the high frequency rolloff of the transmission path manifests itself in the eye diagram as a distortion from its ideal form. Thus, instead of the CRT display showin an eye in which the peak and trough of the eye are precisely half way between the intersections of the waveforms, the peak and trough between two successive intersections are closer, along the time axis, to the earlier intersection than to the later intersection. The use of eye diagrams is discussed at length in chapter 27 of "Transmission Systems for Communications", Bell Telephone Laboratories, 1971, and Ingram, "Analysis And Design of Digital Transmission Systems", Computers and Digital Techniques, 1979, Vol. 2, No. 3.
A known technique for transmitting data that is attracting an increasing amount of attention is the so-called teletext system, in which data is transmitted during the vertical interval of the conventional video signal and is decoded by the television receiver to provide an alphanumeric display. In the transmission of teletext, each line of the vertical interval that is used for data includes not only the data itself (a succession of binary 1's and 0's) but also a clock lead-in sequence of alternating 1's and 0's which precedes the data interval and is used in the receiver to regenerate a clock signal, without which the data cannot be recovered.
It is at present known to monitor the suitability of a transmission path for transmission of teletext by employing a clock regenerator and an oscilloscope. The clock regenerator regenerates the clock signal from the clock lead-in sequence, and the clock signal is applied to the horizontal sweep input of the oscilloscope. The data signal is applied to the vertical deflection input. Clearly, this technique requires the availability of both a clock regenerator and an oscilloscope.